A drill pipe consists of a pipe itself—a rather long component that is almost untreated mechanically, and of interlocking devices with the threaded connections (further referred to as joints or tool joints). The latter are located at both pipe's ends forming either integral joints or permanent connections. This very device will be further denoted by the term “drill pipe”. The term “pipe” will denote a semifinished drill pipe without tool joints.
Tool joints connect pipes into a drill string and represent elements with external tapered thread (referred to as the pipe nipple) or with internal thread (referred to as the tool joint box). As distinct from pipes, tool joints undergo complicated mechanical treatment. Thus it is more economic to produce short joints separately and then weld them to bearing faces. At present, great majority of drill pipes are produced with flash weld tool joints. In order to increase the area of welded connection and decrease tension, pipe ends are made expanded, i.e. upset. Prefabricated joints and pipe ends are joined by means of friction welding (see, for example, Patent of the Russian Federation No. 2291904 acc. to class. C21D 9/08).
Drill pipes outwear in the course of drilling. Certain pipes with flash weld tool joints do not outwear evenly. Endurance capability of the body of a drilling pipe is much higher than that of a weld tool joint. Tool-joint thread wears out much quicker than the pipe body. Therefore, tool joints are designed and fabricated liberal, which allows at least one repair. For repair purposes joints are faced at a height of 20 to 40 mm. Due to the margin, metal consumption is increased, flexural rigidity becomes greater, and the dimensions of tool joints, larger. Revamping of pipes is only possible at pipe yards thus the overall maintenance expenses must include charges for transportation pipes to the yard and back.
At least the thread segments of drill pipes are specially coated. Coating enhances the corrosion stability and wear resistance of pipes and other parts of drilling equipment.
There are, for example, thread connections for oil-well tubing and drill pipes coated with plastic copper-bearing alloy (Patent of the Russian Federation No. 2214478 acc. to class. C23C 26/00). The coating is applied to each reinforced part individually through pressuring the rubbing rod onto the surface of the revolving part, the contact area being simultaneously dampened with a certain liquid. Such a coating enhances corrosion stability and increases resistance to wear of thread connections but the coating process is very complex, which is a shortcoming. Besides, coating parameters differ greatly from part to part since coating is applied to each part individually. Completion of the process is signaled by the appearance of a “harsh whistling sound”, which is a very subjective criterion.
In terms of technical essence and efficiency, the nearest to the proposed solution are the drill pipes, coupling boxes and adaptors that have on their thread connections a two-layer protective coating consisting of a zinc layer (10-14 mcm), which is applied by means of thermo-diffusion galvanizing, and an overlay of phosphate coating (2-3 mcm) (Patent of the Russian Federation No. 38498 acc. to class. F16L15/08). The thermal-diffusion galvanization is most suitable for large-scale production and ensures the coating of appropriate quality. The second phosphate layer is applied so as to enhance the reliability of a 15-mcm thermo-diffusion zinc coating whose continuity is not guaranteed. Such a two-layer coating ensures protection of connecting elements from corrosion whether they are stored or transported. However the coating wears out after first making-up/breakout cycles, with maximum of ten cycles, followed by wearing out of metal thread. Therefore, the coating currently in use does not enhance the endurance capability of tool joints.